A known type of robotic arm is able to follow a path to a work site with the body of the arm remaining on the path. This is known as “tip following”. That is, the body of the arm follows the path taken by the tip, in a snake-like manner. The tip may carry a work tool and, for certain tasks, it is necessary for the arm to follow the path precisely and for the tip to be steady and stable.
Examples of mechanisms that are known that can tip-follow include inchworm style devices, flexible telescoping devices such as that shown in WO 2006/083306 (Choset) and snake-arm robots such as shown in WO 0216995 (Buckingham) and U.S. Pat. No. 6,869,396 (Belson). Inchworm style devices advance by segments expanding and contracting along a path. Flexible telescoping devices advance by extending along a path. A variation on the telescoping theme is to have two flexible telescoping mechanisms one inside the other that alternately extend and rigidise, such as that described in Choset.
Snake-arm robots have multiple articulated segments which are independently controlled. For example, the position of the distal end of each segment is controlled using cables. The segments may have multiple links within a segment, such that each segment can adopt a curved shape. This type of device is described in Buckingham.
Known methods for controlling such a device include the use of a basic leader-follower algorithm, in which the shape of each section or segment of the device is copied to the next adjacent section as the arm advances. It will be appreciated that inchworm and telescoping designs also necessarily use such leader-follower algorithms.
The weakness of this approach is that by copying the motion of a more distal segment to a more proximal segment much of the potential motion capability of the proximal segments is lost. This is because the following segments have to be identical to the leader. In comparison, the known method of controlling non-redundant robots is for an input device to be used to define a desired tip “pose” for the device, and for mathematical algorithms based on Jacobian matrices or inverse kinematics to be solved to derive a unique segment or joint variable solution to achieve the desired tip pose. By “pose” is meant the planar or spatial position and orientation of the tip. For a planar device, the position may be an X, Y location, and the orientation may be about the Z axis. For a fully spatial device, the position may be an X, Y, Z location, with orientations being about the X, Y and Z axes.
Furthermore it is a common requirement for a robot during startup to move from its current undefined position to a known position. This process is often described as homing. With a basic leader-follower method the robot can only start in certain configurations. It would be advantageous to be able to take the current configuration as the homed position and not require the arm to move into a particular shape before commencing work.
It is known that techniques for non-redundant robots may not be generally applied to redundant mechanisms especially where mechanisms are highly redundant.
Therefore there is a need to define methods that go beyond existing leader-follower methods, including the ability for the device to move to a new position in which the motion of its different parts are not necessarily a copy of the previous motion of other parts.